Walking beam type heating furnaces are used in the hot rolling process for heating steel materials such as steel billets, slabs or the like. For supporting and transporting the steel billet, slab or like material to be heated, the furnace has a plurality of rows of skid beams including movable beams and fixed beams. The movable beams periodically repeat vertical and horizontal reciprocating movements to transport the material while alternately transferring the material between the movable beam and the fixed beam.
FIG. 1 shows a skid beam 1 for the walking beam type heating furnace. The beam 1 comprises a hollow skid pipe 10 of heat-resistant alloy and a plurality of skid buttons 12 provided on the pipe 10 as arranged axially thereof at a given spacing. The skid beam 1, which is disposed inside the furnace, has a refractory lining 14 covering the outer periphery of the skid pipe 10 and also covering the skid buttons 12 over the base portion thereof to its upper portion.
With reference to FIG. 6, the skid button 12 of the conventional skid beam is in the form of a block of heat-resistant alloy (such as heat-resistant cobalt cast steel or heat-resistant nickel-chromium cast steel) which is fixedly joined to the skid pipe 10 by welding. Since the interior of the furnace is maintained at a high temperature usually of at least about 1000.degree. C., cooling water is passed through the hollow channel of the skid pipe, thereby preventing the skid pipe from bending, buckling or like formation accompanied by elevated temperature and permitting the pipe to retain flexural strength against the load of the material placed thereon. Further the refractory lining 14, for example, of a castable material covering the surface of the skip pipe suppresses a rise in the temperature of the cooling water and protects the skid pipe from the high-temperature oxidizing atmosphere.
The skid button is influenced by the colling water flowing through the skid pipe and therefore has a lower temperature than the interior of the furnace, with the result that the steel material placed on the top of the skid button is deprived of heat at the portion thereof in contact with the skid button. Thus, the contact of the skid button locally creates a low-temperature portion (a so-called skid mark) in the material, hence the problem of uneven heating. If the uneven heating becomes pronounced, the subsequent rolling step will be seriously affected.
It appears that the skid mark can be eliminated by increasing the height of the skid button and thereby reducing the influence of the cooling water on the top portion of the button. However, an increase in the height of the skid button permits the skid button to have a higher temperature close to the internal temperature of the furnace, consequently reducing the compressive strength of the skid button and allowing the button to undergo compressive deformation because the skid button is usually made of heat-resistant cobalt or nickel-chromium cast steel. The skid button must then be replaced in a short period of time.
It also appears possible to preclude the compressive deformation by giving an increased cross sectional area to the skid button to thereby increase the area of contact between the button and the material to be heated and diminish the compressive load on the button per unit area. Nevertheless, an increase in the contact area correspondingly decreases the surface area of the material to be exposed to the atmosphere of the furnace inside to result in a lower heating efficiency, is liable to entail insufficient heating and an uneven temperature distribution and fails to effectively obviate the drawback.
It further appears possible to use a skid button of sintered ceramic material which has high heat resistivity and high compressive strength at high temperatures. However, while transporting the material to be heated, the skid button is repeatedly subjected not only to a static load but also to a great dynamic load, so that the ceramic skid button, which is low in toughness, is prone to cracking or spalling. Moreover, the ceramic skid button can not be welded directly to the skid pipe and is therefore difficult to attach to the skid pipe. For example, a box-shaped skid button may be fittable in a mount member of heat-resistant alloy, but the button, if having an increased height, is very unstable and is liable to slip off the place, failing to assure the furnace of a stable operation.